Among them are small designer puppies - made popular by celebrities - imported from countries like Lithuania, Hungary and Slovakia, the investigation found.

Before the quarantine laws were relaxed last year, owners needed to vaccinate their pets against rabies and wait six months before taking their animal overseas on a pet passport, meaning only older dogs could come in and out of the UK. But since the changes last January, young puppies aged 15 weeks and above, which have been vaccinated for rabies, can now leave and enter the country on a pet passport.

Dispatches claims the number of dogs brought in on pet passports increased from more than 85,000 in 2011 to nearly 140,000 last year following the changes. It also claims thousand of cheap puppies were being brought in from Eastern Europe, many of them illegally. Importers were found to falsify information in order to get around the rules.

During the documentary, Sharon Edwards, City of London animal health inspector based at the Heathrow Quarantine Centre, said she had detained four times as many illegal dogs since the rule change.

Chief Veterinary Officer Nigel Gibbens denied the change in rules had increased the risk of rabies entering into the UK.

He said in a statement: "There are strict rules in place on movements of pets to protect against rabies entering the UK and to protect the animal's welfare. Those that look to evade these rules are breaking the law and we are determined to crack down on this illegal trade.

"It's crucial that we are told about suspected criminal activity so we can clamp down on it. The ferries, trains and airlines have a legal obligation to check that all pets travelling to Britain comply with the rules and are working with them to make sure these checks are carried out properly.

"Pet owners have the power to tackle this criminal trade by only buying from legitimate breeders. By seeing the puppy with its mother before you buy it, buyers can be assured about its health and welfare."